This invention generally relates to light weight metal alloys and more particularly to lithium-boron-magnesium ternary alloys.
For many uses as structural material it is desirable to have intermetallic alloys which are extremely light in weight, low in atomic number, ductile, malleable, and yet structurally strong and which have high melting points. Beryllium has been used because it meets many of these requirements. However, beryllium metal by nature is too brittle and toxic and is too expensive for general usage. Thus, a search has gone on for other materials which can take the place of beryllium and which do not have the same disadvantages as beryllium.
For instance, binary systems of boron-magnesium, lithium-magnesium, and lithium-boron have been studied for suitability as structural material to replace beryllium. Although the phase diagram of the boron-magnesium system has not been characterized, MgB.sub.2, MgB.sub.4, MgB.sub.6 and MgB.sub.12 do exist and their crystal structures have been identified. However, MgB.sub.2 is undesirable as a structural material because of its high reactivity with air and water, and MgB.sub.4, MgB.sub.6, and MgB.sub.12 are undesirable because they are too brittle to be useful as structural materials.
Freeth and Raynor [J. Inst. Metals, volume 82, page 575 (1953-54)] present a phase diagram for the lithium-magnesium binary system. This phase diagram shows no intermediate phases but rather wide primary solid solution ranges on both the lithium and the magnesium ends of the phase diagram. The alloys on the lithium rich side are undesirable because they are reactive with air and with water. On the other hand, the alloys on the magnesium rich side present a definite fire hazard, making them unsuitable for most applications.
Finally, attempts have been made to prepare metallic lithium-boron alloys. Thus, Markovskii and Kondraskev [Zh. Neorgen Khim., volume 2, pages 34-41 (1957)] and Secrist et al [U.S. Atomic Energy Comm, TID 17, 149 (1962)] and French Patent No. 1,461,878 have all attempted to prepare metallic lithium-boron alloys. In all of these cases, however, dark powders of undetermined composition were obtained. These powders were inorganic compounds which could not be utilized as structural materials because of the lack of the characteristics of metals. It is believed that all of these previous attempts to prepare metallic lithium-boron alloys resulted in the formation of inorganic compounds rather than true metal alloys.
Wang (F. E. ), in U.S. patent application Ser. No. 377,671, filed on Jul. 5, 1973, disclosed a process for the formation of true lithium-boron metallic alloys. Those alloys are light weight, ductile, malleable, and structurally strong. However, those lithium-boron alloys are readily susceptible to air oxidation, thus limiting their usefulness.